A cryotron is a superconductive device in which a changing magnetic field can cause a superconductive element to oscillate between states of low and high resistance. In such a device, current in an input circuit magnetically controls the superconductive-to-normal transition in one or more output circuits provided the current in each output circuit is less than its critical value.
Cryotrons are electronic devices with the potential for use in computer circuits or as on-off elements. However, they cannot be used for switching large currents. There is a need for a superconductive switching device which operates upon application of a relatively small current to direct the flow of a larger current.
The term "superconductivity" refers to a particular state of a material whereby the material exhibits "superconductive" properties. As used herein, the term "superconductivity" designates a material property characterized by zero electric resistivity and, ideally, zero permeability. The existence of superconductivity in a given material at a given time is dependent on the composition and temperature of the material, the electric current that the material is carrying and the strength of any magnetic field to which the material is exposed.
As used herein, the term "normal" refers to the usual resistive state of a superconductive material in which the material does not exhibit superconductive properties. For example, an alloy may exhibit normal conductivity at temperatures above a critical temperature, but is superconductive below the critical temperature. Thus, a material is said to be a "superconductor" when, below the critical temperature, it demonstrates the properties known as superconductivity, in particular, zero electrical resistance.
Superconductor technology offers many possible benefits. For example, a type of battery can be provided because the zero electrical resistance of a superconductor allows for the near permanent storage of electrical energy in the form of a permanently circulating current. Once a current is generated in a superconductive circuit, it will continue to flow with little or no loss of energy in what is termed a persistent mode.
The critical temperatures have, until recently, generally been between zero to about twenty degrees Kelvin. Recent developments have demonstrated the existence of "high" temperature superconductive materials which exhibit superconductive properties at about 100 degrees Kelvin. As these developments are proceeding rapidly, these high temperature superconductors will probably reach even higher temperatures. A major research goal is the development of a material that exhibits superconductive properties at or near room temperature.
At any temperature below the critical temperature for the material, the material will retain its superconductive properties unless it is exposed to a critical current (I) or a critical magnetic field strength (B) or a combination of both. The critical values for the current and the magnetic field are temperature dependent, with the values increasing as the temperature decreases. For example, when the temperature of the material is very close to but still less than the critical temperature, the current needed to revert the material to the normal state will be minimal. As a result, in order for a superconductor material to carry an appreciable current, the material is cooled to a temperature considerably below its critical temperature.
An I versus B curve may be constructed for a specific temperature for a particular superconductor. A frequently used temperature is 4.2 degrees Kelvin, the boiling point of liquid helium. Liquid helium is often used as the coolant to maintain superconductors at the low temperatures required.
One end of the I versus B curve is defined by the critical current needed to cause the superconductor to revert to its normal state when the magnetic field strength is zero. The other end is the critical magnetic field strength needed to cause the superconductor to revert to its normal state when the current is zero. The curve which connects these two points is known as the I versus B curve. Below the curve, the material is a superconductor. Above the curve, the material is in its normal resistive state.
When the characteristic material passes through the curve it undergoes a transition. The term "transition" relates to the state of a superconductor when it passes from the normal state to the superconductive state, or vice versa. The time duration of the transition state can be considered to exist between the beginning and the end of the transition wave form.
One way to cause a superconductor to go through a transition is to apply a switching current. The term "switching current" means of an applied current which causes an element of an opening switch to interrupt at a predetermined maximum voltage and predetermined frequency under defined operating conditions.
This switching current can be used to operate a switching device. The term "switching device" refers to a device which opens, or closes, or either one alternatively, one or more electric circuits.
Superconductors often have difficult stability problems. One problem arises if a small portion of a superconductor carrying a current reverts to the normal state. This can occur if the particular portion is exposed to heat and the temperature thereof exceeds the critical temperature. With the extremely low temperatures employed, such difficulties are encountered frequently.
When such a "hot spot" goes normal, it will continue to heat up because of the sudden electrical resistance encountered. In turn, this causes adjoining areas to also go normal and thereby the hot spot perpetuates itself throughout the superconductor. To rectify the situation, the current must be shut off and the superconductor cooled before continuing.
In the prior art, superconductive switching devices commonly suffer from such poor stability characteristics. For example, poor stability characteristics are commonly associated with prior art direct interrupt superconductivity switch devices. One reason for this instability is the need to operate these devices near their critical limits.
Also, in the prior art, superconductive switching devices frequently require application of a large switching current to produce a direct interrupt and associated switching from superconducting-to-normal in output circuit(s).
There is need for a superconducting switching device which has better stability characteristics than, for example, direct interrupt superconducting switches, and which can be caused to open with only a small current unlike cryotrons. The present invention provides such a device.